The present invention relates to the art of limiting circuits. As used herein, a "limiting circuit" is one whose output signal is constant or substantially constant for all input signals above a critical value.
Limiting circuits have many applications and are especially useful where there is a necessity to achieve waveform shaping, spike suppression or limiting of the dynamic operating range of certain succeeding circuitry. A specific example of a use for such a circuit is in a control system of an automatic process manipulator to limit the dynamic range of the signal applied to subsequent servo amplifiers. A versatile and effective limiting circuit should have several characteristics. First, the limiter should have a convenient means for optimally setting the upper and lower limits; this feature is especially useful during field installation of large, complicated control systems. Also, it is desirable to be able to adjust the limits either statically or dynamically in both positive and negative going directions. Further, it is important to provide hard limiting as opposed to soft limiting. These terms are applied generally when the limiting circuits are unidirectional current conducting devices such as diodes. Generally, limiting by use of diodes is termed soft because of the gradual turn on characteristics of the diodes. The term hard limiting thus refers to the degree to which the errors introduced by this characteristic of diodes are reduced.
Presently known circuits for providing limiting include conventional, as well as zener, diodes used either directly in shunt with a load or as feedback elements in operational amplifier circuits. For example, one simple diode limiter includes a pair of diodes connected back to back in shunt with a load. This limiter is restricted to applications which are fixed to single diode voltage drop levels. Also, the limiting provided is termed soft because of the gradual turn-on characteristics of the diodes. Various diode limiters which produce temperature compensated limiting are known but these limiters also suffer to some degree from the same soft limiting characteristics.
Limiters using operational amplifiers with diodes in the feedback loops have better driving characteristics than the simple diode limiters, but the limiting level is fixed at the zener diode voltage, and such limiters still produce soft limiting due to the gradual turn-on characteristics of the diodes and the limited loop gain available. Transistor limiters are sometimes used. Here, limiting is obtained by driving the transistor into saturation and cutoff. However, this circuit is complicated to implement practically because of the transistor's biasing requirements. Also, this circuit has restricted applications.
A more complicated limiting circuit is that described in the U.S. Pat. No. 3,697,780 issued to Michael et al. This circuit includes a pair of operational amplifiers with feedback networks associated therewith. The input signal to be controlled is applied to the input of the first amplifier. A first reference signal representative of a desired lower limit is applied to the inputs of both of the amplifiers. A second reference signal representative of the upper limit controls the feedback network of the second amplifier. The output signal from the second amplifier is representative of the amplitude of the input signal as long as this signal is above the lower and below the upper limit. However, this circuit is quite complicated and is restricted in use to input signals of only one polarity.